The original IBM-PC computer had a 5.25 inch flexible disk drive for removable mass storage. The data format used by that original 5.25 inch flexible disk drive has become a de facto standard for both flexible disks and data storage tapes. Personal computers which are compatible with the IBM-PC/AT computer typically have several higher capacity 5.25 and 3.5 inch flexible disk drives which use a data format which is compatible with the original IBM-PC 5.25 inch flexible disk data format. These computers typically have one flexible disk controller which is shared by all the flexible disk drives. It is common to use that flexible disk controller to also control a data storage tape drive. Originally, both flexible disk drives and compatible tape drives used magnetic heads having a single gap for reading, writing and data erasing plus a separate pair of gaps for trimming the edges between tracks.
Conventional magnetic heads have a gap for writing dam longitudinally along the surface of magnetic recording media. Higher bit densities require shorter read gap lengths. However, in general, if a write gap becomes too short, the fields from the short write gap may not extend far enough into the magnetic medium to effectively overwrite or erase previously recorded data. One solution to higher bit densities is to use a separate gap for erasing or to use a head assembly with one section optimized for writing and a separate section optimized for reading.
Another solution to higher bit densities is to orient magnetic polarity changes vertically within the magnetic medium (perpendicular to the surface) rather than longitudinally (parallel to the surface). This is typically accomplished with special media having a high permeability layer below the data layer. Flexible disk drives and compatible controllers are now available which can accommodate perpendicular recording read/write heads. Examples of commercially available controllers include the 82077SL disk controller from Intel Corporation and the 8477 disk controller from National Semiconductor Corporation. These controllers are actually dual, mode controllers in that they can be used with perpendicular recording read/write heads in a first mode or with conventional heads in a second mode.
Flexible disk controllers designed for perpendicular recording are designed specifically for drives in which the magnetic medium is rotating in a single direction. In addition, the controllers are designed specifically for a head with a separate erase gap which always precedes the writing portion of the head so that the head can erase before writing. These controllers simultaneously erase during every write operation.
In contrast, tape drives typically record in a serpentine fashion in which the tape movement relative to the head is bidirectional. If a bidirectional tape drive used a head with a single erase gap offset from the write gap, data written in one of the two directions would immediately be erased by the trailing erase gap. A head could be fabricated with both leading and trailing erase gaps but this would make the head much more expensive. There is a need for tape drive systems with low cost heads which are appropriate for bidirectional tapes and which can still share standard commercially available flexible disk drive controllers. In addition, there is a need for higher density flexible disk drives with conventional media.
Data storage disks and tapes are typically divided into fixed size data blocks called sectors. These sectors are typically further subdivided into fields for synchronization and error detection. In addition, there are areas between fields and sectors to accommodate variations in speed from drive to drive. These areas between fields are called GAPs. In this specification, to help distinguish between a gap in a magnetic head and a media format GAP, head gaps are lower case and media GAPs are capitalized.